1. Boot down the phone
2. Keep holding down Volume Up button.
3. Put a USB cable that is connected to a PC in to phone.
4. Once it boots into the desired mode, release the Volume Up button.

Extra: Here's the how-to video.

Download mode may appear prior to fastboot mode.

1. Boot down the phone
2. Keep holding down Volume Up button.
3. Put a USB cable that is connected to a PC in to phone.
4. Continue holding Volume Up button throughout download mode until fastbood mode appears and you can release the Volume Up button.

Extra: Here's the how-to video.